1. Field of the Invention
The present invention relates to a thermal image transfer recording medium which is particularly suitable for a thermal image transfer recording method by use of a thermal head.
2. Discussion of Background In a conventional thermal image transfer recording method, image recording is performed by the steps of bringing an image receiving sheet into pressure contact with a thermal image transfer recording medium which comprises a substrate and a thermal transfer ink layer formed thereon in such a manner that the image transfer sheet comes into contact with the thermal transfer ink layer; applying pulse-like signal voltages corresponding to the images to be recorded to a thermal head which is out in contact with the back side of the substrate, opposite to the thermal transfer ink layer with respect to the substrate, whereby heat is generated in the thermal head in accordance with the signal voltages applied thereto; and fusing or subliming the thermal transfer ink layer imagewise by the heat generated in the thermal head, so that images are transferred to the image receiving sheet and image recording is performed.
As the substrate for use in the conventional thermal image transfer recording media for the above-mentioned thermal image transfer recording method, plastics films such as a polyethylene torephthalate film are in general use.
When a thermal image transfer recording medium with a substrate made of such a plastics film is actually used in thermal image printing, the plastics film is easily fused and adheres to a thermal head during image printing operation, so that occasionally the running or transportation of the thermal image transfer recording medium is stopped or the thermal image transfer recording medium is broken by the sticking of the recording medium to the thermal head. Such a trouble is referred to a sticking phenomenon.
In order to prevent the occurrence of such a sticking phenomenon, there have been proposed varieties of heat resistant protective layers to be provided on the back side of the substrate of such a thermal image transfer recording medium, with which side a thermal head comes into contact.
For example, heat resistant protective layers, made of silicone resin, epoxy resin, melamine resin, phenolic resin, fluoroplastics, or aromatic polyamide, have been proposed in Japanese Laid-Open Patent Applications 55-7467, 60-201989 and 63-172688. These heat resistant protective layers are made of any of the above-mentioned resins, and therefore have the shortcoming that the lubricating properties thereof are insufficient to completely prevent the occurrence of the sticking phenomenon.
In Japanese Laid-Open Patent Application 61-143195, there is proposed the use of a silicone graft or block acrylic copolymer in a heat resistant protective layer. By use of the copolymer, the sticking phenomenon can be prevented, but the film formation properties of the copolymer are so poor that it could occur that the heat resistant protective layer peels away from the substrate. Furthermore, the heat resistant protective layer is easily worn, so that improper running on transportation of the thermal image transfer recording medium is caused.
In Japanese Laid-Open Patent Applications 1-221281 and 1-234292, there are proposed a heat resistant protective layer made of ethyl cellulose resin with the addition of silicone oil thereto, and a heat resistant protective layer made of an acetate-group-containing cellulose ester resin with the addition of silicone oil thereto. In these heat resistant protective layers, the lubricant properties thereof can be improved by the addition of silicone oil and therefore the occurrence of the sticking phenomenon can be reduced. However, the silicone oil component is apt to be transferred to the thermal head and therefore the thermal head is easily smeared with the transferred silicone oil component, so that the printing quality is significantly lowered.
In Japanese Laid-Open Patent Application 4-126294, there is proposed a heat resistant protective layer made of a mixture of an acryl silicone graft polymer and a reaction product of an amino-modified silicone oil and a polyfunctional isocyanate. In this heat resistant protective layer, the transfer of the silicone component to the surface of the thermal head is minimized, but the film formation performance of the heat resistant protective layer and the adhesion between the substrate and the heat resistant protective layer are insufficient, so that this heat resistant protective layer easily peels away from the substrate. Furthermore, the abrasive resistance of this heat resistant protective layer is not sufficiently improved.
In Japanese Laid-Open Patent Applications 5-58067 and 5-162472, there is proposed the use of a silicone polyvinyl butyral copolymer or a silicone-modified cellulose acetate phthalate for use in a heat resistant protective layer. A heat resistant protective layer comprising any of the above materials is improved with respect to the film formation properties, but the heat resistance thereof is insufficient for use in practice, so that a fused material detached from the protective layer adheres to the thermal head, resulting in the lowering of the printed image quality.
Furthermore, cross-linking reactions by use of isocyanate, melamine or epoxide with remaining hydroxyl groups in a resin employed in a heat resistant protective layer are conventionally often utilized in order to improve the heat resistance and film formation properties of the heat resistant protective layer. This method, however, has the shortcomings that a curing process is required after the coating of a heat resistant protective layer formation liquid on a substrate and that such a heat resistant protective layer formation liquid has the shortcoming that its pot life is short.